It is known in the art that isoparaffins and monoolefins can be reacted in the presence of an HF catalyst to produce higher molecular weight hydrocarbons. This alkylation process has found widespread use for the production of high octane gasoline. It is further known in the art that isoparaffins can be alkylated optimumly, separately by butylenes and by propylenes. It has also been proposed in the art to recycle the unreacted isoparaffin which is separated from the alkylate in an isoparaffin stripper. It is further known in the art that a portion of the effluent of a first alkylation reactor can be either recycled directly to this reactor or to another HF alkylation reactor in which the same alkylation reaction is carried out as in the first alkylation reactor. In the latter case, the two reactors in which the same alkylation reaction occurs are quasi connected in series.
The introduction or recycling of hydrocarbon effluent into the alkylation zone is reported to improve the octane number of the alkylate produced. Recycling total reactor hydrocarbon effluent from a HF butylene/isoparaffin alkylation reactor has the advantage of increasing the isoparaffin butylene ratio without increasing the load on the isostripper. However, this procedure has the disadvantage over recycling pure isoparaffin of increasing the production of heavy hydrocarbons such as fractions with 12 carbon atoms or more per molecule. It would thus be desirable to have an alkylation process available that can be operated at high isoparaffin/olefin rate, and at the same time produces only a minimum amount of heavy hydrocarbons and does at the same time not increase the utilities required, particularly for the isoparaffin stripper.